It is a constantly circulating fluid within the body that provides nutrition, oxygen to all the organs of the body. It is composed of various types of cells and proteins that are suspended in it. This makes blood thicker than pure water. It is composed of mainly three types of cells – red blood cells (transportation of oxygen), white blood cells (fight infections) and platelets (help blood clot). Every human has about five liters of blood within his/her body which is constantly replenished over a period of time. Another main constituent of blood is plasma which makes up about half the volume of blood. It contains a protein that helps in clotting blood and transporting nutrients amongst other functions. Blood is transported through the body via blood vessels such as arteries, veins and capillaries.